Baking of NOS tubes in an oven

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I was pointed out a link to an article about "baking" NOS tubes in a normal household oven. The idea of this is, to do something similar as burn in, in an easier way.

If you would like to know about getter technology, the article of Werner Espe is a classic.

A large text part by Morgan Jones, you find in the link below here. I went to Morgan Jones' lectures on a tube festival, and he sure is one of the best experts I know. Please note, I have no relation with this website, it is only I link, I give here: http://www.r-type.org/articles/art-223.htm

Here is a link to an thread in diyaudio. If you read the texts there, you will see the technical level of a discussion about this subject. Oh, and the baby picture you see there, was stolen from my website.

Classic, electrical burn in, has quite some effects:

Why (or when) is burn in needed for NOS?

It is needed when NOS tubes right out of the box, test as if they are a little but used, and have some small grid current.

It is not needed, if the tubes test flawless, right away. In that case you can skip the burn in. I have seen that many times with cellophane sealed Telefunken ECC801S. Today you don't find those any more sealed, but I remember those days. Whereas sealed Telefunken ECC81 sometimes need a burn in badly. Also I once had a sealed pair on RE134 from 1935. Opened by me in 2010, so 75 years after production. They were spot on by the data sheet immediately, and did not change a bit while letting them run for a while. I had many NOS C3g and C3m. Same here also. They are spot on, and do not change. But this is rare, and not a must.

Actually, today most NOS have collected some very small fractions gas after all those decades. It's really very little, and not a problem, but yes it is there.

What causes gas residue in NOS tubes?

It's a thing called "Channels". The electric wires have to go through the glass. Thin wires are pulled from thicker wires, which are pulled from a chunk of metal. From all the pulling, there are very fine scratches along the surface, and it is possible, these form a channel. Now, the wire is not just melted inside the glass as is. At the inside the wire is nickel, at the outside it is copper, and these two parts are sealed (welded) together with a third part, which is a special alloy, with the same temperature coefficient as glass. Very technical wires. So the channel, if there is one, is sealed by those two welds. But nothing is perfect, and even when it is really extremely little, some molecules can (and will) leak in, over the decades. It should be clear that good quality feed through wires is the #1 item here. On the other hand it should also be clear that at the end of tube mass production, there were many last time buy options, and these tubes simply did not get the best materials ever. Not saying these are bad tubes, but these need a lot more burn in than tubes which are twice as old.

What does the getter do at storage?

Well it does nothing. The getter reaches full activity at 180° C. Below that, there is partial activity, and concluding from the experiments of Morgan Jones, there is activity at 130°C as well, but so little, it takes 21 hours to clean the tube. Whereas during production, the tube is cleaned within seconds at getter flashing. At room temperature the getter will get white of course at low or no vacuum, but it can not maintain or restore high vacuum.

What is the getter made of?

We talk here about the getter flash on the glass. The active substance is metallic Barium, which is highly active at high temperature. It reacts with anything possible.

What is the cathode active layer made of?

Also metallic Barium. Since this can not exist in open air, it has to be formed, created, after and during vacuum pumping, with a process we call tube activation. The end of this, is a very thin layer of active Barium.

Questions....

It is particularly here, where I put a question mark at tube baking. It is proven, this improves vacuum, because the #1 indicator for this, GRID CURRENT, reduces sharply, as Morgan Jones demonstrated with EF184 tubes. There is good and bad in this. Also the CATHODE is absorbing this gas, because it has also a layer of Barium, and it is "only" 100° C at baking. This is called sometimes cathode poisoning, but poisoning is something else. Poisoning is done rather by metal ions. But the result is the same: Loss of emission.

To prevent this, the cathode should be heated during the baking, but that brings the next problem. If heated, it MUST carry some minimum DC current too. And then... the baking is not needed, the tube gets warm by itself. We call it electrical burn in.

Conclusion:

The gas residue will be absorbed by the Barium of the getter, and by the Barium of the cathode as well.

This process will give initial improvement, by removing the gas, if there is some. It probably compromises the cathode to some extend. So the process makes sense when you have 100's of low cost, tubes, some of which have grid current (=gas), and you want a quick result for the whole batch. The batch will be better as it was before, but just not with ultimate result.

For expensive tubes, which you would not have laying around by the 100's anyway, I would advise to go for classic burn in, with plate current, not compromising the cathode quality.